1. Field of the Invention
The present invention relates to taste enhancers and taste attenuators. The present invention provides a variety of uses for a series of sodium channel blocking pyrazinoylguanidine compounds represented by formula (I) defined herein to modify the taste of various foodstuffs on the palate. These compounds can be used as taste modifiers in various foodstuffs.
2. Description of the Background
Humans, as a living organism, require nutrition like any other organism. As such, they also require a way to distinguish between safe food items and dangerous food items. This ability has evolved within us as a species, and it has resulted in the pleasure and displeasure we feel when eating certain foods. Bitter and sour foods that we find unpleasant, while salty, sweet, and meaty tasting foods generally provide a pleasurable sensation. The five specific tastes received by gustatory receptors are salt, sweet, bitter, sour, and umami, which means “savory” in Japanese. Both salt and sour taste mechanisms detect the presence of sodium chloride in the mouth in different ways. The detection of salt is important to many organisms, but specifically mammals, as it serves a critical role in ion and water homeostasis in the body. Because of this adaptive nature, salt elicits a pleasant response in most humans. Sour taste can be mildly pleasant in small quantities, as it is linked to the salt flavour, but in larger quantities it becomes more and more unpleasant to taste. This is because increasing sour taste can signal over-ripe fruit, rotten meat, and other spoiled foods, which can be dangerous to the body because of bacteria which grow in such mediums. As well, producing a sour taste, acids (H+ ions) cause serious tissue damage. The bitter taste is almost completely unpleasant to humans.
Arguably the simplest receptor found in the mouth is the salt (NaCl) receptor. An ion channel in the taste cell wall allows Na+ ions to enter the cell. This ion flow depolarizes the cell, and opens voltage-regulated Ca2+ gates, flooding the cell with Ca2+ ions and leading to neurotransmitter release. This sodium channel is known as ENaC (Epithelial Sodium Channel) and is composed of three subunits. ENaC can be blocked by the drug amiloride in many mammals including humans. The sensitivity of the salt taste to amiloride in humans, however, is less pronounced, leading to conjecture that there may be additional receptor proteins besides ENAC that may not have been discovered yet.
Sodium Taste and ENaC Pharmacology
Na+ transport across epithelia, including lingual epithelia, involves the passive flux of Na+ from the luminal compartment (oral cavity for dorsal lingual epithelia) into the epithelial cells (for taste, receptor cells in taste buds and some surrounding non-taste epithelia) through apical membrane ENaC. Na+ is then pumped across the basolateral membranes of the epithelial cells by the Na+—K+-ATPase (Verrey et al. (2000)). Transcelluar Na+ transport can be inhibited at the cell apical membrane by adding amiloride or one of its analogues to the luminal solution, thereby reducing the Na+ permeability of ENaC (Verrey et al. (2000)). For the species in which transepithelial ion transport has been studied across the dorsal lingual epithelium, viz., dog (DeSimone et al. (1984); Simon and Garvin (1985), Mierson et al. (1985)), rat (Heck et al. (1984); Mierson et al. (1988); Garvin et al. (1988); Gilbertson and Zhang (1998)), rabbit (Simon et al. (1986)) and hamster (Gilbertson and Zhang (1998)) the trans epithealial Na+ transport paradigm appears to apply. The most compelling evidence that ENaC or some ENaC variants are indeed Na+-specific salt taste receptor proteins is that the taste nerve response to NaCl is significantly inhibited by amiloride and its analogues in a variety of species. Taste responses to NaCl recorded in the afferent chorda tympani or in the nucleus of the solitary tract of rat (Schiffman et al. (1983); Heck et al. (1984); Brand et al. (1985); Hill and Bour (1985); Scott and Giza (1990); Yoshii et al. (1986); Ninomiya and Funakoshi (1988); St. John and Smith (2000)), hamster (Hettinger and Frank (1990)); some mouse strains (Ninomiya et al. (1989)), and gerbil (Schiffman et al. (1990)) are significantly inhibited by amiloride without effect on responses to stimuli of other taste modalities. Amiloride sensitivity is observed in single chorda tympani units of the chimpanzee that respond strongly to Na+ and L+ salts, but not in units sensitive to both Na+ and K+ (Hellekant et al. (1997)). Whole cell patch clamp studies on isolated rat and hamster taste buds show that amiloride blocks a Na+ current across taste cell membranes, consistent with a role for ENaC in Na+ taste reception (Avenet and Lindemann (1988; 1991); Gilbertson et al. (1992)). Taken together the data are consistent with the conclusion that ENaC is the Na+ specific salt taste receptor in many species including many rodents and non-human primates. In rats the amiloride-sensitive part of the response accounts for 75%-80% of the total response to 100 mM NaCl. ENaC likely plays a role in the more complex mechanism of human salt taste (1995)).
Sour taste signals the presence of acidic compounds (H+ ions in solution). There are three different receptor proteins at work in sour taste. The first is a simple ion channel which allows hydrogen ions to flow directly into the cell. The protein for this is ENaC, the same protein involved in the distinction of salt taste (this implies a relationship between salt and sour receptors and could explain why salty taste is reduced when a sour taste is present). There are also H+ regulated channels present which are regulated by acid-sensing ion channels (ASIC's).